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Nanotube Yarns Generate Electricity When Stretched

Billy Hurley, Digital Editorial Manager

Scientists at The University of Texas at Dallas and South Korea’s Hanyang University have developed tiny, high-tech yarns that generate electricity when stretched or twisted. The nanoyarns, constructed from hollow carbon nanotubes, create current when coated with an ionically conducting material — even a simple mixture of table salt and water.

When the carbon nanotube yarn is inserted into an electrolyte bath, the yarns are charged by the electrolyte itself. Unlike traditional capacitance harvesters, no external battery, or voltage, is needed.

Dr. Carter Haines, a lead researcher and associate research professor at UT Dallas, sees short-term and long-term applications for the nanoyarn. Haines spoke with Tech Briefs about the range of possibilities, from ocean-wave energy harvesting to sensors in smart textiles.

Tech Briefs: What does a nanotube yarn look like?

Dr. Carter Haines: We make these small-diameter fibers by growing the nanotubes upright in a “forest.” We take the same silicon wafer you use to make electronics, add a catalyst layer, and put the wafer inside an oven. The nanotubes grow vertically from that substrate. If you pull out a sheet of nanotubes and twist it just like you would with sheep’s wool, you can make a yarn that’s comprised purely out of nanotubes.

Tech Briefs: How does a nanotube yarn work? How is electricity generated?

Haines: We’re exploiting a change in capacitance. When we dip the yarn inside an electrolyte, it charges up as a supercapacitor. By stretching the yarn, we change the capacitance. Basically, we’re squeezing the nanotubes — and the ions on the nanotube surface — closer together, giving us greater voltage out from the yarn.

From left: Dr. Carter Haines BS'11, PhD'15, Dr. Shi Hyeong Kim and Dr. Nai Li of the Alan G. MacDiarmid NanoTech institute at UT Dallas are lead authors of a study that describes carbon nanotube yarns that generate electricity when they are stretched or twisted. (Credit: UT Dallas)

Tech Briefs: How much electricity can be produced?

Haines: Right now, we’re working with very tiny yarns, so the actual energy coming out is small. But if we normalize per weight of the yarns that we’re using as a harvester, we can get up to about 250 watts per kg of yarn as peak power.

Tech Briefs: What can be realistically powered with this amount of electricity?

Haines: The hope right now is that we can replace other types of very-small-scale generators, and also be able to harvest energy in places where it’s really, really hard to have a large generator. For instance, you may want to have different sensor nodes for an Internet-of-Things application. Let’s say you want to embed multiple sensors in your shirt and not have all these running on separate batteries that need to be replaced. We think that having very-small-scale harvesters that are flexible — and not bulky like conventional electromagnetic generators — is a realistic application in the near term.

We’ve sewn these yarns inside a T-shirt and shown that we can use them as breathing sensors. As someone is breathing in and out, the shirt is expanding and contracting, and harvesting energy from that.

Tech Briefs: What are some long-term application possibilities?

Haines: In the long term, we’d like to be able to harvest large amounts of electricity. There’s a lot of energy in ocean waves that’s basically being wasted. We’re not harvesting any appreciable amount of that to be used in the power grid.

Tech Briefs: Was this tested in the ocean?

Haines: We actually put one of these into nanoyarns into the ocean. We were just stretching and releasing the yarn. One side was tethered was to the bottom of the ocean floor, and the top side was attached to a balloon floating on the surface. We showed that we could harvest electrical energy out as the waves came and moved that balloon around.

Tech Briefs: What’s next regarding the development of the technology?

Haines: Right now one of the main difficulties is that the nanotubes are very expensive to make by this process. Some of our work moving forward will be to learn how to lower the cost and find other materials that will allow us to get the same level of energy harvesting.

What do you think? Will ocean-wave energy harvesting succeed? Will nanoyarns lead to new wearables? Share your comments below.

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A study, co-authored by Haines, was published in the Aug. 25 issue of the journal Science.